The invention relates to a process for preparing a solution that is active as a thixotropic agent and contains urea urethanes, in which monohydroxyl compounds are reacted with diisocyanate and the monoisocyanate adducts formed are further reacted with diamines in aprotic solvent in the presence of a lithium salt to form urea urethanes and to the use of the solution, in particular, for imparting thixotropic properties to coating compounds, such as, for Example, solvent-Containing, solvent-free and aqueous paints and lacquers, PVC plastisols and coatings based on epoxide and based on unsaturated polyester resins.
To control the rheology of liquid coating compounds, organically modified bentonites, silicic acids, hydrogenated castor oil and polyamide waxes are predominantly used. A disadvantage of these substances is that they are generally dry solids that have to be dispersed by means of solvents and shear forces to form semi-finished products or be introduced into the liquid coating compound by systematic temperature control. If these temperatures are not maintained, crystallites, which may result in defects in the coating, occur in the finished coating compound.
The general disadvantage of these rheological auxiliaries used at present is that they result in turbidities and clouds (haze) in clear, transparent coatings. In addition, handling dry, powdered products causing dusts during processing is undesirable.
Other achievements relating to rheology control were disclosed in European Patent Application EP-A-0 198 519. In the latter, an isocyanate is reacted with an amine in the presence of paint resin solutions to form a urea that forms needle-shaped crystals in very finely dispersed form. These paint binders modified in this way are offered as rheology-control agents and sag-preventing binders, as so-called xe2x80x9csag control agentsxe2x80x9d.
The disadvantage of these products is the result of the fact that they are always bound to the binder in which they have been prepared and do not permit any subsequent universal correction of finished coating agents.
European Patent EP-B-0 006 252 describes a process for preparing a thixotropic agent that eliminates some of the abovementioned disadvantages by describing urea urethanes that are prepared in aprotic solvents in the presence of LiCl by reacting isocyanate adducts with polyamines. The disadvantage of the products prepared in this way is the undefined structure of said urea urethanes due to the preparation process. Although isocyanate monoadducts are described, there are in fact no monoadducts at all in this case, as becomes clearly apparent from the Example, but mixtures of different adducts. In the process described, one mole of a diisocyanate is first reacted with one mole of a monoalcohol. This process partly produces the desired NCO-functional monoadducts, but also diadducts with no NCO-functionality. In addition, a proportion of the monomeric diisocyanate remains unreacted.
The proportions of these different compounds may vary, depending on the accessibility of the NCO group and the reaction conditions applied, such as temperature and time. The reason for the greatest disadvantage is, however, that all these adducts prepared in this way contain fairly large amounts of unreacted diisocyanate that, during the further reaction with polyamines in the presence of lithium chloride, results in uncontrolled chain extension of the urea urethane and in polymeric ureas. These products then have a tendency to precipitation and can be kept in solution only with the greatest difficulty.
The object of the present invention is therefore to find a process to generate thixotropic agent of a defined structure and consequently to enable a greater storage stability over several months of solutions prepared in this way and to enable a more reliable application of the products.
Surprisingly, it was found that this problem solution can be achieved in that monohydroxyl compounds of the general formula (I)
Rxe2x80x94OHxe2x80x83xe2x80x83(I)
in which R is n-alkyl or isoalkyl containing 4 to 22 carbon atoms, cycloalkyl containing 6 to 12 carbon atoms, aralkyl containing 7 to 12 carbon atoms or a radical of the formula CmH2m+1 (Oxe2x80x94CnH2n)xxe2x80x94or CmH2m+1 (OOCxe2x80x94CvH2v)xxe2x80x94, where m=1-22, n=2-4, x=1-15 and v=4 or 5, are reacted with a 1.5 times to 5 times molar excess of toluene diisocyanate, the unreacted part of the toluene diisocyanate is removed from the reaction mixture and the monoisocyanate adduct obtained is reacted with diamines of the general formula (II)
H2Nxe2x80x94Rxe2x80x3xe2x80x94NH2xe2x80x83xe2x80x83(II)
where Rxe2x80x3=xe2x80x94CyH2y with y=2-12 and/or with 
and/or with 
where Rxe2x80x2xe2x80x3=CH3xe2x80x94 or H, in the presence of a lithium salt.
The molar excess of toluene diisocyanate is preferably 2-4 mol, particularly preferably 2-3 mol. Preferably, the toluene diisocyanate isomers contain a 2,4-isomer proportion of 50-80%.
After completion of the reaction of the alcohol Rxe2x80x94OH with parts of the available diisocyanate to form the isocyanate monoadduct, the unreacted excess of the diisocyanate is removed from the reaction mixture, preferably by vacuum distillation under mild conditions.
LiCl or LiNO3, but preferably LiNO3, may be used as lithium salts. The solids content of the urea urethane solutions generated in this way is 5-80%, preferably 20-60%, particularly preferably 25-50%. The reaction of the monoadducts with the diamine takes place in a polar aprotic solvent, such as, for Example, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-butylpyrrolidone or comparable alkylpyrrolidones.
The amount of lithium compounds is 0.2-2 mol, preferably 0.5-1.5 mol, particularly preferably 0.75-1.25 mol of lithium, relative to the amine equivalent of the diamine used.
The use of LiNO3 in contrast to LiCl is particularly advantageous since chloride ions have a disadvantageous effect on coating compounds and promote the corrosion of the metallic bases to which the thixotropic coating compounds are applied.
The urea urethanes prepared in this way do not contain either free isocyanate groups or free amino groups. They are accordingly physiologically safe. Furthermore, no adverse side reactions occur with binders or fillers. The storage stability of these urea urethane solutions prepared in this way is extraordinarily high and is certainly 6 months or more at, normal storage temperature.